linux-kernel - Re: WMI and Kernel:User interface

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Date:   Wed, 10 May 2017 15:50:44 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Mario.Limonciello@...l.com
Cc:     Darren Hart <dvhart@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Pali Rohár <pali.rohar@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org
Subject: Re: WMI and Kernel:User interface

On Wed, May 10, 2017 at 3:02 PM,  <Mario.Limonciello@...l.com> wrote:
> So here's a "more" realistic scenario:
>
> OEM has support through a WMI function to control keyboard backlight timeouts
> and intensity.  That same WMI function also can support turning on/off an individual
> USB port.  Backlight timeouts are done by setting the first argument to "1" and USB
> port control is done by setting first argument to "2".
>
> Some userspace app is developed that can control both of these functions through
> the chardev.  Later an enterprising young kernel developer realizes that backlight
> control should be done through a platform driver instead.
>
> They write a platform driver to do it, and add a filter to block "1" arguments from
> userspace.  Now if the userspace app tries to call the chardev with the "1" argument
> some error code is returned indicating this request is not supported.
>
> The result is the userspace app broke, but it broke because the kernel is supporting
> the method in a much smarter and more scalable way.

There's another possibility: the filter could intercept the "1"
argument and change the brightness.

FWIW, I think that almost anything the kernel did with a real WMI API
would be better than the ugly things that drivers like dcdbas allow.

--Andy